Dual-gridded reflector antennas consist of two separate polarized reflectors gridded in orthogonal directions, typically referred to as vertical and horizontal directions. Each reflector is illuminated by a single feed or a feed array, which is polarized along the direction of the respective grid. Typically, the two reflectors are nested, one behind the other, to provide beams with different shapes on, for example, vertical and horizontal polarization. For instance, the front reflector would reflect horizontally polarized signals from one feed while being nearly transparent to the orthogonal vertically polarized signals from the other feed. The rear reflector would reflect the vertically polarized signals from the other feed which pass through the front reflector. Cross-polarized radiation currents on a reflector from its respective feed are attenuated due to the grids. This component is referred to as the "right reflector" cross polarization. However, cross polarization currents from the feed induced on the orthogonally polarized reflector are not attenuated since the grids are aligned in the cross-polarized direction. This component, which is dependent on the relationship of the two reflectors, is called the "wrong reflector" cross polarization. Typically, the feeds are displaced from the focus of the orthogonally polarized reflector so the cross-polarized radiation is scanned away from the coverage area. Accordingly, the "wrong reflector" cross polarization depends on the feed separation and, therefore, is traded against mechanical complexity of the antenna construction. The "right-reflection" cross-polarization depends upon the grid parameters and so is easily controlled. The present invention relates to the scanning and suppression of the "wrong-reflection" cross-polarization.
Traditional dual-gridded reflector antennas fall into two categories. In the over-under configuration, the front and back reflectors have offset directions aligned as shown by the front view of the configuration shown in FIG. 2a. However, the respective offsets are different as shown by the side view of the configuration in FIG. 2b, resulting in a feed separation. In the rotated-offset configuration, the front and back reflectors have equal offsets and the offset directions are rotated with reference to each other, as shown by the front and side views in FIGS. 3a and 3b resulting in a feed separation. In both of these configurations, however, the direction of the feed separation (the line joining the two feeds) is either in the east-west direction or the north-south direction. Since the "wrong reflector" cross polarization radiation is scanned along the direction of the feed separation, in both of these configurations the scan would be in either the east-west or north-south direction. In satellite applications where coverage areas are very large in the north-south or east-west directions, traditional dual gridded reflector configurations required a large feed separation which results in a complex antenna configuration. In many applications, however, it is preferable to scan the cross polarized radiation in a direction other than pure north-south or east-west. Thus, there exists a need for a dual gridded reflector antenna configuration which allows the cross polarized radiation to be scanned in any given direction to improve cross polarized radiation performance and simplify the mechanical package.